In recent years, attention has been given to hybrid vehicles that use an engine and an electric motor in combination. The hybrid vehicle can run using only an electric motor without using an engine and thus can perform EV drive when a battery is in a sufficiently charged state.
Japanese Patent Laying-Open No. 2003-343304 has disclosed a hybrid vehicle in which an engine starts when a large change occurs in accelerator position during EV driving. This can increases an acceleration response.
In a certain hybrid vehicle that has been studied, a battery has a large capacity and is externally chargeable so that an engine operation ratio can be low and refueling is less required. This kind of vehicle is referred to as an “externally chargeable hybrid vehicle” in this description.
As compared with an ordinary hybrid vehicle allowing only refueling, the externally chargeable hybrid vehicle has a battery of a higher output and a higher capacity for increasing an EV drive region so that the EV drive is positively performed when stored electricity is left in the battery. Thereby, it is intended to reduce the fuel consumption and an amount of carbon dioxide emissions.
For example, a user who uses the externally chargeable hybrid vehicle for going a relatively short distance to work may charge a battery every night at home. In this case, the engine starts only when the quantity of stored electricity in the battery becomes substantially zero during a weekend long-distance drive, or a driver depresses an accelerator pedal to increase a load of the vehicle from a small load.
For increasing the effect of reducing the amount of carbon dioxide emissions in the externally chargeable hybrid vehicle, it is necessary to use the battery power with a higher priority than the fuel, as compared with the ordinary hybrid vehicle. However, when a higher priority is assigned to the use of the battery power, the acceleration response may deteriorate. Description will now be given on the deterioration of the acceleration response.
FIG. 14 is a waveform diagram for illustrating an example in which the acceleration response deteriorates.
As shown in FIG. 14, a threshold of engine start is set with respect to a required drive output PD (i.e., a power required for driving a vehicle). Thus, when required drive output PD increases to or above a high load exceeding the threshold, the engine starts.
For using the battery power on a priority basis as compared with the ordinary hybrid vehicle, the start threshold of the required drive output may be set as illustrated in FIG. 14 for starting the engine when the vehicle load increases. Thus, the start threshold of the required drive output starting the externally chargeable hybrid vehicle may be increased as compared with that of the ordinary hybrid vehicle so that for retarding the start of the engine.
However, in the vehicle having the threshold thus changed, the engine start will be delayed from time t31 to time t32 when a high load is suddenly required during a low speed driving, e.g., in an operation of fully accelerating the vehicle from a stopped state.
Even when an accelerator position Acc becomes maximum, required drive output PD does not immediately increase above the threshold. This is because required drive output PD depends on a product of the required drive torque primarily determined based on accelerator position and a vehicle speed. Therefore, when the vehicle speed is low even though the accelerator position Acc becomes maximum, required drive output PD does not exceed the threshold so that the engine start is delayed until the vehicle speed increases.
This phenomenon slows the response to a user's acceleration request. However, it is not preferable that the externally chargeable hybrid vehicle has a remarkably slow acceleration response as compared with the ordinary hybrid vehicle.